How many one-gallon plastic milk jugs filled with water, lined up back to back, must a bullet penetrate in order to qualify it as sufficient penetration to incapacitate a bad guy when shot from a handgun 6 feet away?

The caliber is not the issue, the barrel is not the issue, revolver or auto, but the number of milk jugs. Is it 1, 2, 3, 4 or 5?